Devices comprising a communication system transceiver are known from the state of the art, for example for enabling a GSM (Global System for Mobile communications), a US-TDMA (US Time Division Multiple Access or IS-136), a CDMA (Code Division Multiple Access) or a WCDMA (Wideband CDMA) communication of the device via some communication network. Further, a receiver operating at a different frequency band than such a communication system can be for example a satellite positioning system receiver, like a GPS (Global Positioning System) receiver of a GPS system or a DVB-T (Digital Video Broadcast-Terrestrial) receiver of a DVB-T system. A communication system transceiver operating at a first frequency band and a receiver operating at a second frequency band can also be implemented together in a single device, for instance in a mobile phone, in a PC (personal computer) or in a laptop.
The performance of the receiver may be degraded, however, during time intervals in which wideband noise in the second frequency band reaches the receiver, since this wideband noise may reduce the signal-to-noise ratio (SNR) of the received signals significantly.
The wideband noise can be generated in particular by a communication system transceiver integrated in the same device as the receiver, or by a communication system transceiver external to this device.
In a GPS system, for example, several GPS satellites that orbit the earth transmit signals which are received and evaluated by GPS receivers. All GPS satellites use the same two carrier frequencies L1 and L2 of 1575.42 MHz and 1227.60 MHz, respectively. The modulation of these carrier frequencies L1 and L2 is illustrated in FIG. 1. After a phase shift of 90 degrees, the sinusoidal L1 carrier signal is BPSK (bi-phase shift key) modulated by each satellite with a different C/A (Coarse Acquisition) code known at the receivers. Thus, different channels are obtained for the transmission by the different satellites. The C/A code, which is spreading the spectrum over a 1.023 MHz bandwidth, is a pseudorandom noise sequence which is repeated every 1023 chips, the epoch of the code being 1 ms. The term chips is used to distinguish the bits of a modulation code from data bits. In parallel, the L1 carrier signal is BPSK modulated after an attenuation by 3 dB with a P-code (Precision code), and the L2 carrier signal is BPSK modulated with the same P-code before an attenuation by 6 dB. Before transmission, the two differently modulated parts of the L1 carrier signal are summed again. The L2 carrier signal carries currently only the P-code. The P-code is much longer than the C/A code; Its chip rate is 10.23 MHz and it repeats every 7 days. In addition, the P-code is currently encrypted, and for that reason it is often referred to as P(Y)-code. Decryption keys needed for using the P(Y)-code are classified and civil users cannot access them. Therefore, only the L1 carrier C/A code is usable in civil GPS receivers.
Before the C/A-code and the P(Y)-code are modulated onto the L1 signal and the L2 signal, navigation data bits are added to the C/A- and P(Y)-codes by using a modulo-2 addition with a bit rate of 50 bits/s. The navigation information, which constitutes a data sequence, can be evaluated for example for determining the position of the respective receiver. The L1 signal which is received at a receiver is further modulated due to the Doppler effect and possibly due other higher order dynamic stresses.
The reception bandwidth of a GPS receiver receiving the modulated satellite signals is related to the reception code. For example, if GPS is based on the L1 carrier C/A code, then the signal requires a frequency band of 1575.42 MHz+/−5 MHz. If a P-code capable receiver is used, then the reception band of the GPS receiver is much wider, it is likely to be 1575.42 MHz+/−24 MHz. The actual used GPS reception bandwidth is further related to the actual implementation, and thus the previously mentioned bandwidths are presented for demonstration purposes. The mentioned GPS bandwidth will thus be used in the following only by way of example.
The GPS standard is currently under modernization. One of the main components of the modernization consists in two new navigation signals that will be available for civil use in addition to the existing civilian service broadcast of the L1-C/A code at 1575.42 MHz.
The first one of these new signals will be a C/A code located at 1227.60 MHz, i.e. modulated onto the L2 carrier frequency, and will be available for general use in non-safety critical applications. The new civilian signal at L2, referred to as “L2CS”, will generally be characterized by a 1.023 Mcps (mega chips per second) effective ranging code having a Time Division Multiplex of two ½ rate codes. The L2CS signal will be BPSK modulated onto the L2 carrier, along with the P(Y)-code. This C/A code will be available beginning with the initial GPS Block IIF satellite scheduled for launch in 2003.
The second one of the new signals will be using a third carrier frequency L5 located at 1176.45 MHz. The L5 carrier frequency will be modulated with C/A codes, more specifically with a CL code of 767,250 chips and a CM code of 10,230 chips. The L5 signal will provide a 10.23 Mcps ranging code, wherein it is expected that improved cross correlation properties will be realized. The L5 signal will be message based. It will include an I (In-phase) channel carrying 10-symbol Neumann/Hoffman encoding and a Q (Quadrature) channel carrying 20-symbol Neumann/Hoffman encoding. The I and Q channels will be orthogonally modulated onto the L5 carrier. The L5 signal falls into a frequency band which is protected worldwide for aeronautical radionavigation, and therefore it will be protected for safety-of-life applications. Additionally, it will not cause any interference to existing systems. Thus, with no modification of existing systems, the addition of the L5 signal will make GPS a more robust radionavigation service for many aviation applications, as well as for all ground-based users, like maritime, railways, surface, shipping, etc. The new L5 signal will be available on GPS Block IIF satellites scheduled for launch beginning in 2005.
At the current GPS satellite replenishment rate, all three civil signals, i.e. L1-C/A, L2-C/A and L5, will be available for initial operational capability by 2010, and for full operational, capability approximately by 2013.
Measurements show that if no measures are taken, the SNR of a GPS signal received by a GPS receiver degrades by about 2 dB in case a GSM transceiver implemented in the same device uses for transmissions a single slot TX (transmission) mode, and by about 3 dB in case the GSM transceiver implemented in the same device uses for transmissions a dual slot TX mode.
In particular communication systems operating in the 1900 band, like GSM1900, which are widely referred to as PCS (Personal Communication System), and communication systems operating in the 1800 band, like GSM1800, which are widely referred to as DCS (Digital Communication System), will generate wideband noise in this GPS L1 band of 1575.42 MHz+/−5 MHz, when C/A code supported GPS is used. When new L2 and L5 frequency GPS signals are used, then lower frequency GSM signals, i.e. GSM900 and GSM800, will generate the same wide band noise problem as GSM1800 to the L1 GPS signal.
The GPS receiver, however, requires a sufficient SNR of received satellite signals for being able to correctly acquire and track the signal based on its C/A code and thus to make use of its content. It is better for the performance of the GPS receiver to receive signals with a particularly low SNR than not to receive any signal at all during short time intervals.
Typically in spread spectrum systems, the AGC (Automatic Gain Control) tunes the received information signal level for A/D (analog to digital) conversion based on the noise level. In normal operation conditions, the noise is coming from background noise, which has a constant power level. The problem arises when the noise level rises rapidly and the AGC tries to adjust an incoming signal to a certain appropriate level for an A/D conversion. A fast varying high noise level can cause saturation in the A/D converter and the amplitude of the signal is clipped. If the signal is clipped in conversion, some information signal is lost and thus the receiver performance is degraded.
Also external interferences can block a GPS receiver operation completely, in case multiple communication system transceivers are transmitting in the same area at the same time.
For illustration, FIG. 2 shows a communication system with a base station 10 of a communication network, a first mobile station MS1 11 comprising a GSM transceiver and a second base station MS2 12 comprising a GSM transceiver and in addition a GPS receiver. The first and the second mobile station 11, 12 may exchange signals with the base station in uplink and downlink transmissions. During the uplink transmissions of either of the two mobile stations 11, 12, wideband noise is generated in the GPS frequency band, which may disturb the performance of the GPS receiver of the second mobile station 12. In order to notice a noise rise in the GPS receiver of the second mobile station 12 during a GSM transmission by the first mobile station 11, the first mobile station 11 has to be near to the second mobile station 12. This is due to the fact that the propagation loss, i.e. the attenuation on the air interface, is increased, when the distance between the transmitter and the receiver is increased.
The same problem as in the case of GPS may occur, for example, when a Galileo receiver is used instead of a GPS receiver. Galileo is a European satellite positioning system, for which the beginning of commercial operations is scheduled for 2008. Galileo comprises 30 satellites, which are distributed to three circular orbits to cover the entire surface of the Earth. The satellites will further be supported by a worldwide network of ground stations. It is planned that Galileo will provide ten navigation signals in Right Hand Circular Polarization (RHCP) in the frequency ranges 1164-1215 MHz, using carrier signals E5a and E5b, 1215-1300 MHz, using a carrier signal E6, and 1559-1592 MHz, using a carrier signal E2-L1-E1. Similarly as with GPS, the carrier frequencies E5a, E5b, E6 and E2-L1-E1 will be modulated by each satellite with several PRN codes spreading the spectrum and with data. Thus, GSM transmitters may equally generate wideband interferences in frequency bands employed by Galileo.
Obviously, the performance of a receiver due to transmissions by a communication system transceiver may equally be degraded in a similar situation in case of another type of a communication system transceiver and/or another type of a receiver.
In U.S. Pat. No. 6,107,960, a method is proposed for reducing cross-interferences in a combined satellite positioning system receiver and communication system transceiver device. A control signal is transmitted from the communication system transceiver to the satellite positioning system receiver, when the communication transceiver transmits data over a communication link. The control signal causes the satellite positioning system signals from satellites to be blocked from the receiving circuits of the satellite positioning system receiver, or to be disregarded by the processing circuits of the satellite positioning system receiver. In case the of a GSM transceiver using a single slot TX mode, the resulting performance degradation is always 0.6 dB=(10*log 10(⅛).
This method is only able to improve the performance of the satellite positioning system receiver, however, if the interfering signals are generated by a communication system transmitter integrated in the same device as the satellite positioning system receiver.
A similar performance degradation as in a satellite positioning receiver system may occur as well in a DVB-T receiver system.
DVB-T was first adopted as a standard in 1997, and is currently rapidly expanding in Europe, Australia and Asia. DVB-T offers about 24 Mbit/s data transfer capability to a fixed receiver, and about 12 Mbit/s to a mobile receiver using an omnidirectional antenna. Some distinguishing technical features of DVB-T include the following: DVB-T offers a net bit rate (R) per frequency channel in the range of about 4.98 to 31.67 Mbit/s and operates with a channel separation of 8 MHz in the UHF range of 470-862 MHz. In the VHF range of 174-216 MHz, the channel separation is 7 MHz. Single frequency networks can be used. DVB-T uses a Coded Orthogonal Frequency Division Multiplex (COFDM) multi-carrier technique with QAM (Quadrature Amplitude Modulation), 16 QAM or 64 QAM carrier modulation. The number of sub-carriers can be between 1705 (2 k) to 6817 (8 k). An inner forward error correction coding (FEC) uses convolutional coding with rates of ½, ⅔, ¾, ⅚ or ⅞, while an outer coding scheme uses Reed-Solomon (204,188,t-8) coding. Outer bit-interleaving uses convolutional interleaving of depth 0.6-3.5 ms. For 8 k mode, the duration of the symbol part is 896 micro seconds and for 2 k mode 224 micro seconds. The actually seen DVB-T symbol length is the symbol duration and a guard time which can be ¼, ⅛, 1/16 or 1/32. DVB-T was developed for MPEG-2 Transport stream distribution, but it is capable as well of carrying other types of data. For example, DVB-T can provide a broadband, mobile wireless data transport for video, audio, data and Internet Protocol (IP) data.
DVB-T is scalable, with cells sizes ranging from, for example, 100 km down to picocells of e.g. tens to hundreds of meters. The capacity is very large. For example, 54 channels can be supported, each running at 5-32 Mbit/s. One time slot packet is 188 (204) bytes long. Due to the large number of sub-carriers, the symbol time can be made very long. For example, for the 8 k sub-carrier case, the symbol time is on the order of 1 millisecond. A guard interval is inserted before each symbol.
Thus, while DVB-T is well suited for providing digital video streams, DVB-T can be used as well to provide high speed data streams for other types of applications, such as interactive services, Internet access, gaming and e-commerce services. As can be appreciated, for interactive and other services to be provided, a return link or channel is required from the user back to some server or other controller. One example of such as a system is MediaScreenJ by Nokia. This device provides an LCD display screen for displaying information received from a DVB-T downlink, and includes a GSM function having a transmitter to provide the return link or channel.
When using such a constellation, a problem arises because the lower end of the GSM transmission band begins at 880 MHz, while the upper end of the received DVB-T frequency band ends at 862 MHz in European channel allocation. Thus, transmitted energy from the GSM band can leak as wideband interference into the DVB-T receiver, resulting in errors in the processing of the received data.
It should be noted that while the foregoing presentation has concentrated on specific DVB-T frequencies and the European GSM system, the same problems can arise in other locations where DVB-T has been specified for use. For example, in the United States of America, digital television is referred to as ATSC (Advanced Television Systems Committee), and currently the FCC has allocated the frequency bands of 764-776 MHz and 794-806 MHz for Digital Television (DTV) broadcasts. One U.S. cellular transmission band, which is already occupied, has been established from 824-849 MHz. As can be noted, the upper boundary of the DTV band of 806 MHz is separated from the lower end of the cellular transmit band of 824 MHz by only 18 MHz, about the same separation that is seen in the DVB-T/GSM embodiment described above.
It has been proposed that when a DVB-T receiver and a GSM transmitter are combined in a single device, the GSM transmitter may notify the DVB-T receiver about its transmissions, and the DVB-T receiver integrates an incoming signal only when GSM transmission is not active. In a DVB-T system, the symbol length is long compared to the length of a GSM burst and thus multiple GSM bursts can occur during one DVB_T symbol time.
This approach is only suited to improve the performance of the DVB-T receiver, however, if the interfering signals are generated by a GSM transmitter integrated in the same device as the DVB-T receiver.